Cutting processes involve three fundamental factors: high accuracy, high efficiency, and low cost. Of the three factors, high efficiency is considered to be attained by increasing cutting speed, and therefore, means for such increase has become of interest. With an improvement in cutting speed, cutting efficiency also improves. However, tool life becomes shortened, resulting in increased tool costs. Another problem with the shortened tool life is that increased frequency of tool replacement adversely affects productivity. Thus, realization of highly efficient processing still encounters difficulty.
In particular, since high-precision shaped products have recently been demanded, dies are also required to have high accuracy. Thus, instead of addressing electric discharge techniques which have conventionally been employed, researchers have now focused attention on the direct cutting method for producing dies, as this method is believed to provide high-precision dies at high production rate.
For example, Patent Document 1 (JP-A HEI 11-170102) discloses the following techniques in relation to improved cutting. Namely, the percentage of CBN sintered material contained in a CBN sintered tool is made 75% or more; cutting is performed at a rate of at least 1,500 m/min; a plurality of CBN sintered tools are employed to form a face milling cutter; the cutting speed of the face milling cutter is not less than 1500 m/min.; and cutting is performed with a per-flute feed rate of the CBN sintered tool of 0.2 to 0.4 mm/rev, that is, 0.2 to 0.4 mm per revolution of the face milling cutter. According to Patent Document 1, even when the cutting speed is increased, the above features effectively prevent deterioration of tool life.
Patent Document 2 (JP-A 2003-268486) discloses a hot working tool steel containing 0.28 to 0.55 mass % of C, 0.15 to 0.80 mass % of Si, 0.40 to 0.85 mass % of Mn, not more than 0.020 mass % of P, not more than 0.018 mass % of S, 2.5 to 5.7 mass % of Cr, 1.4 to 2.8 mass % of Mo, 0.20 to 0.90 mass % of V, 0.01 to 1.65 mass % of W, 0.03 to 0.89 mass % of Co, 0.01 to 1.65 mass % of Ni, and the substantial balance of Fe and unavoidable impurities; with the amounts of N, Ti and B contained in the unavoidable impurities being restricted to be 0.009 mass % or less, 0.003 mass % or less and 0.012 mass % or less, respectively, with the cleanliness of nonmetallic inclusions as specified by JIS being dA of 0.005% or less and d(B+C) of 0.020% or less, and with the crystallographic orientation of martensite having undergone heat treatment being 17 to 33%. It further discloses that use of the hot working tool steel can improve machinability, significantly prolong tool life and reduce variation in tool life when the steel material is subjected to a machining process for forming a die through direct cutting and, when subjected to ultrafine cutting, can provide excellent finish surfaces, thereby shortening the time required for lapping.
Also, Patent Document 3 (JP-A HEI 8-188852) discloses a die formed from a material having a composition of 0.25 to 0.45 wt % of C, 0.05 to 0.6 wt % of Si, 0.2 to 0.8 wt % of Mn, 4.0 to 6.0 wt % of Cr, 1.0 to 3.0 wt % of Mo, 0.3 to 1.0 wt % of V, 0.005 to 0.040 wt % of Al, 0.001 to 0.004 wt % of S, and the balance of Fe and unavoidable impurities and having hardness of HRC 41 to 45. Patent Document 3 also discloses that when the die having the above composition is produced through a method including die sinking and plastic forming, in which the material is subjected to die sinking to thereby form a die, and each of the round corner portions of the die-sunk surface of the die is subjected to plastic forming under a pressurizing device having a radius of curvature smaller than the radius of the corner so as to attain a surface plastic forming of a total equivalent strain of 5% or less, the production cost of the die is as low as in the case of producing a conventional hot forging die from a JIS SKT4 or SKD61 steel, and that the die exhibiting excellent durability can be produced under a working environment better than that under which conventional dies are produced.
However, according to the technique described in Patent Document 1, the requirements of 75% or higher CBN sintered material accounting the entire material of the tool and a cutting speed of 1,500 m/min or more are very special conditions which cannot be fulfilled by generally employed machine tools, and in addition, the suggested tool is quite expensive and impractical.
Patent Document 2 discloses investigations on the material, but fails to discuss processing conditions so as to identify specific, optimal conditions.
Patent Document 3 discloses investigations on the material and also discloses a method to impart a compressive stress to a corner portion. However, it fails to disclose specific conditions of the cutting process.
The direct cutting process has the following problems in relation to the cutting conditions. To cope with a variety of shapes, deep milling is required, with a length of tool extension prolonged. Under conventional situations where spindle speed and feed rate are determined on a trial and error basis, when the length of tool extension is increased, optimal conditions cannot be obtained. For example, in usual practice where a spindle speed is set to a maximum possible level, and the feed and the depth of cut (pitch) are calculated on the basis of the required surface roughness, if the calculated feed and depth of cut values are employed as they are, undesirable phenomena, such as chatter of the tool, are caused. In such a case, undesirable phenomena actually occur and correction of the values is needed. Conventionally, reduction of feed rate has usually been recommended for achieving an excellent finish. In reality, in order to determine the conditions under which the best finished surface can be obtained, iterations of trials and errors have been needed, which is time-consuming.
In addition, when a die is produced through conventional direct cutting at high cutting speed, the finish is poor, and therefore, a polishing step is indispensable. The polishing step requires a considerable workload, increasing the die production cost and extending the production time. Moreover, polishing has been a major cause of defects, as it is generally performed by hand. Under such circumstances, research efforts have centered on development of an improved direct cutting process capable of providing a sufficient surface finish level which eliminates or simplifies the polishing step.
The present invention has been conceived in view of the foregoing, and the objects of the invention include provision of a forging die production method which permits high-speed cutting during production of the forging die, ensures tool life, eliminates the need for polishing and thus realizes overall high efficiency in production, provision of a forging die and provision of forged articles produced through forging by use of the forging die.